The present invention relates to a mold apparatus for producing a hollow-structured article by an injection molding method in which a pressurized fluid is introduced into a molten resin injected in a cavity of a mold and the volume of the cavity is expanded during the molding, and an injection molding method using the above mold apparatus.
For example, JP-A-3-9820 and JP-A-5-84786 disclose an injection molding method in which a molten resin is injected into a cavity formed in a mold, a pressurized fluid is introduced into the molten resin in the cavity while the distance of the cavity in the mold opening direction is increased for expanding the cavity volume, and the molten resin is cooled and solidified in the cavity, thereby to form a hollow-structured injection-molded article having a high hollowness ratio.
In the injection molding method disclosed in JP-A-3-9820, a mold 2 is composed of a fixed mold member 2a and a slidable mold member 2b. The volume of a cavity 8 is expanded by gradually sliding the mold member 2b while introducing a low pressure fluid into a molten synthetic resin 3 which has been injected into the cavity 8.
In the mold disclosed in JP-A-5-84786, a movable core 2 constituting a cavity wall surface on the movable mold member 1 side is provided such that it goes through the movable mold member 1 and is movable in the mold opening and closing directions. A mold opening and closing mechanism is connected to a support frame 3 fixed to the movable mold member 1, and a movable core movement mechanism 4 for moving the movable core 2 in the mold opening and closing directions is supported at a support frame 3. In the mold disclosed in JP-A-5-84786, the volume of a mold cavity 5 is expanded by the movement of a movable core 2 without moving the movable mold member 1.
When an injection-molded article is required to have high appearance characteristics, or when an injection-molded article having a thin rib or embossing as a design or for adding a certain function is produced, it is required to improve the imprintability of the design or embossing of the cavity wall surface to an injection-molded article surface. For this purpose, when a molten resin is injected into a cavity, it is required to press the molten resin on the cavity wall surface at a high pressure or to introduce a high pressure fluid into a molten resin in the cavity. However, when a high pressure fluid is introduced into the molten resin in the mold disclosed in JP-A-3-9820, a molten resin leaks through the parting faces of the mold members 2a and 2b, and a burr is liable to occur on the injection-molded article surface. Moreover, it is made difficult to control the sliding of the mold member 2b. JP-A-3-9820 is silent concerning any specific means of energizing the mold member 2b toward the fixed mold member 2a and of sliding the mold member 2b.
In the mold disclosed in JP-A-5-84786, the movable core 2 constitutes a cavity wall surface on the movable mold member 1 side. That is, a mold cavity 5 is composed of a cavity wall surface of each of the fixed mold member 6 and the movable core 2. The pressure caused by a molten resin injected into the cavity is exerted on the fixed mold member 6 and the movable core 2. It is therefore necessary to apply pressure to the movable core 2 with the movable core movement mechanism 4 so that the movable core 2 can withstand the above pressure. It is therefore necessary to constitute the movable core movement mechanism 4 of a drive block 11 and a following block 8 or of a drive screw 25. As a result, there is a problem in that the structure of movable core movement mechanism 4 is complicated and that the production of the mold apparatus requires an additional cost. Further, when the movable core movement mechanism 4 is composed of a fluid cylinder, it is required to press a molten resin on a cavity wall surface at a high pressure as described previously. For allowing the fluid cylinder to generate the resistant force which can withstand this pressure, there is another problem in that it is required to increase the capacity of the fluid cylinder. JP-A-5-84786 describes that the mold opening and closing mechanism is connected to a support frame 3 fixed to the movable mold member 1, while nothing specific is described concerning the so-described mold opening and closing mechanism. Further, the mold disclosed in JP-A-5-84786 has no structure in which the movable core 2 is moved with the mold opening and closing mechanism.